A Traceroute will trace the route of a packet on your computer to a host. It will show how many hops that the packet takes to reach the host and how long each hop takes.
In the packet that is sent is a time limit value "time to live" (TTL), this time limit is designed to be exceeded by the first router and then it can be used to determine the hop time which returns a Time Exceeded message. The packet is then re sent again with an increased time limit value so that it can then reach the second router where it will also then send another Time Exceeded message, this continues back and forth until it reaches the destination. The Traceroute knows when it has reached the destination because a port number is also included in the packet that is outside of the normal range of port numbers, this then means that a Port Unreachable message is returned which signals to the Traceroute that this is the end and can measure the final hop time for the destination. Because of all of this backwarding and forwarding measuring hops is why that it can take a while for a Traceroute to complete.